A standard utility knife has an elongated handle having an outer end and a longitudinally extending flat blade secured in the handle and projecting outwardly from the outer end. The blade has a transverse outer edge and a longitudinally extending side or front cutting edge meeting at a point. Such a knife is useful for opening packages, including cartons and bags, and is typically carried in the pocket or tool belt of the person using it.
In order to protect the user from injury by the cutting edge and point it is known from German patent 3,116,354 and U.S. Pat. No. 4,086,698 to form the handle with a longitudinally outwardly open and longitudinally extending guide slightly offset laterally from the cutting edge of the blade. A longitudinally extending pin has an outer end carrying a bumper or shield and is longitudinally displaceable in the guide between an extended positions with the outer end lying longitudinally outward of the point and a retracted position with the outer end longitudinally inward of the point. A spring is braced between the pin and the handle and urges the pin into the extended position.
The problem with this system is that it is relatively easy to accidentally push in the guard. Thus if the knife is in a pocket and the user reaches in to grab it, the knife or guard can be pushed to retract the guard and allow the blade to come into direct contact with the user or the user's clothing.
U.S. Pat. No. 142,942 describes a similar knife for use by a cobbler. A spring-loaded mechanism is provided in the handle to set the projection of the blade so as to limit the depth of cut, but it does not provide any protection against injury from the blade. The blade can be retracted into the handle or ejected from it by special maneuvers.
Another knife is known from German 3,433,286 which has a U-shaped guard for the blade. This guard, as in German 3,116,354, is spring loaded so it can be pushed back in by careless handling. Thus it provides little real safety for the user of the knife.
In German 3,540,026 there is a pivotal blade guard of elastic material. The guard can be locked in position by a relatively complex maneuver so that it is likely never to be locked, thereby affording little protection to the user.